1. Field of the Invention
The present invention relates to an optical filter switching apparatus for switching optical filters for use in an optical instrument such as a closed circuit television (CCTV) camera. In particular, the present invention is directed to an optical filter switching apparatus for switching optical low pass filters (OLPFs) in an optical instrument as the illumination changes from daytime to nighttime and vice versa.
2. Description of the Related Art
Surveillance cameras such as CCTV cameras are photographic apparatuses for taking pictures of a specific place or an object remote from the camera and sending the pictures to a CCTV or a monitoring system, thereby allowing a person to confirm the safety or condition of the object or area under surveillance without going to the remote location. The use of surveillance cameras is diversified; such surveillance cameras are used not only for maintenance of public security and prevention of crime in buildings, parking places, apartment complexes, etc. but also as vehicular security devices.
In general, surveillance cameras are classified into three categories: color camera, black and white (B/W) camera, and color and black-white camera depending on color reproducibility.
Color and black-white cameras typically reproduce a color video image using the surrounding light intensity at daytime when the light intensity is usually sufficient without using separate illumination. Meanwhile they reproduce a black and white video image using a plurality of infrared rays light emitting diodes (IR LEDs) for irradiating infrared rays from opposite sides of a lens toward the front to replenish the light intensity of the lens in a dark place where an object is hardly recognizable to the human eye or at nighttime.
Therefore, color and black-white cameras use the visible wavelength band (for example, 400 nm to 700 nm) in daytime to take advantage of the human visual system, while they use an infrared wavelength (for example, 900 nm) at nighttime. However, image pickup devices of CCD (Charge-Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) type, which are employed as photoelectric conversion sensors that convert light into an electric signal in the color and black-white cameras, use only one wavelength regardless of daytime or nighttime. During the daytime both the visible and infrared rays are input through the lens causing a focus distortion. The focus distortion results from a variation in the focusing distance due to the difference in the wavelength between visible and infrared rays.
In order to prevent the focus distortion, conventional color and black-white cameras are provided with a daytime optical filter for filtering out infrared rays and transmitting visible rays only. Color and black-white cameras also have a nighttime optical filter for transmitting infrared rays only to the front of the image pickup device. Therefore, the filters can be switched with one another whenever day and night change.
FIGS. 1 and 2 illustrate a conventional color and black-white surveillance camera 1, which employs an optical filter switching apparatus 10.
The color and black-white surveillance camera 1, has a lens 50, an optical filter switching apparatus 10, an image pickup device 60 and a video image signal processing section 70.
The optical filter switching apparatus 10 comprises an optical filter switching assembly 20, a drive control section 45, a microprocessor 47, and a memory 49.
Referring to FIG. 2, the optical filter switching assembly 20 comprises: a front plate 40 located to the side of the lens 50 (not shown), and an optical filter holding section 21 movably mounted on the front plate. The optical filter holding section 21 further comprises a daytime optical filter 30 and a nighttime optical filter 29. The optical filter transfer section 28 of the optical filter switching assembly 20 transfers the optical filter holding section 21 left and right so as to move the daytime optical filter 30 and the nighttime optical filter 29 between a daytime optical filter setting position (FIG. 3A) and a nighttime optical filter setting position (FIG. 3B). When the daytime optical filter 30 and the nighttime optical filter 29 are positioned in the positions shown in FIGS. 3A and 3B, respectively, the image information passes between the lens 20 and a image pickup device 60, respectively. The rear plate 31 assembled with the front plate 40 covers the optical filter holding section 21 and the optical filter transfer section 28.
The daytime optical filter 30 filters out the light of infrared band from image information input through the lens 50 and transmits the light in the visible band of the image information, whereas the nighttime optical filter 29 transmits the light in the infrared band of the image information.
The optical filter transfer section 28 comprises a stepping motor 35, a gear rod 38, a transfer propelling guide 23, and a guide rod 33. The stepping motor 35 is mounted on an anchoring bracket 36 located at a side of the lower part of the front plate 40. The gear rod 38 is coaxially connected to a shaft of the stepping motor 35. The transfer propelling guide 23 projects from the bottom of the optical filter holding section 21. A downwardly opened internal screw 24 engages the gear rod 38. A guide rod 33, which is inserted into a plurality of alignment grooves 27 formed on a plurality of support projections 26, guides the movement of the support projections 26. The support projections 26 are preferably formed on the top of the optical filter holding section 21 in such a way that the support projections are alternately laterally projected.
The microprocessor 47 generates a control signal for moving the optical filter holding section 21 to the daytime optical filter setting position or the nighttime optical filter setting position in accordance with an externally input mode change signal, or a change in illumination between daytime and nighttime, such as, a power level of a video image signal output from the video image signal processing section 70.
The video image signal processing section 70 converts an electric signal, which is output through the image pickup device 40, into a video image signal and outputs the video image signal to a CCTV or a monitoring system (not shown). The image pickup device 40 converts the image information passing through the daytime optical filter 30 or the nighttime optical filter 29 into an electrical signal.
The drive control section 45 controls driving of the stepping motor 35 of the optical filter switching assembly 20 according to the control signal from the microprocessor 47.
The memory 49 stores position information about a position to which the optical filter holding section 21 is moved by the stepping motor 35, the gear rod 38, and the transfer propelling guide 20, such as, the nighttime optical filter setting position and the daytime optical filter setting position. At the time of manufacturing, the position information for the optical filter holding section 21 being located at the daytime optical filter setting position is recorded into the memory 49.
The surveillance camera 1 constructed as described above operates as follows. Firstly, it is assumed that the surveillance camera 1 is operating in the daytime mode. When a user changes the operation mode from the daytime mode (i.e., color mode) to the nighttime mode (i.e. black and white mode), or when the power level of a video image signal outputted from the video image signal processing section 70 decreases to a predetermined power level of a nighttime video image signal, the microprocessor 47 recognizes the current operation mode as the black and white mode. The microprocessor 47 generates a control signal to move the optical filter holding section 21 from the daytime optical filter setting position to the nighttime optical filter setting position in accordance with the position information of the optical filter holding section 21 recorded in the memory 49.
According to the control signal from the microprocessor 47, the drive control section 43 generates a drive signal so as to rotate the stepping motor 35 of the optical filter switching assembly 30 by a predetermined rotational amount in a direction, for example, clockwise.
As the stepping motor 35 rotates, the gear rod 38 coaxially connected to the shaft of the stepping motor 35 rotates clockwise, so that the transfer driving guide 23 is moved to the right by the downwardly opened internal screw 24, which engages the gear rod 38.
As a result, the optical filter holding section 21 moves to the right from the daytime optical filter setting position shown in FIG. 3A along the guide rod 33 held in the alignment grooves 27 on the support projections to the nighttime optical filter setting position shown in FIG. 3B.
After the optical filter holding section 21 is positioned at the nighttime optical filter setting position, the microprocessor 47 outputs the position information of the optical filter holding section 21 and records it into the memory 49. The video image signal processing section 70 converts an image signal, which is outputted through the image pickup device 60 by way of the lens 50 and the nighttime optical filter 29, into a video image signal and outputs the video image signal to a CCTV or a monitoring system.
Thereafter, when the user changes the operation mode of the camera 10 from the nighttime mode to the daytime mode, or when the power level of the video image signal output from the video image signal processing section 70 increases to the predetermined power level range for the nighttime video image signal, the optical filter holding section 21 is moved to the daytime optical filter setting position. This happens when the microprocessor 47 recognizes the current operation mode as the daytime mode and generates a corresponding control signal. Upon receiving the corresponding control signal, the drive control section 45, stepping motor 35, gear rod 38 and transfer driving guide 23 perform the above-mentioned operations in reverse, thereby moving the optical filter holding section 21 from the nighttime optical filter setting position to the daytime optical filter setting position.
After the optical filter holding section 21 is positioned at the daytime optical setting position, the microprocessor 47 stores the position information of the optical filter holding section 21 in the memory 49. The video image signal processing section 70 converts the image information, which is output through the image pickup device 60 by way of the lens 50 and the daytime optical filter 30, into a video image signal and outputs the video image signal to the CCTV or the monitoring system.
However, in the conventional surveillance camera 1, such a change for a daytime to nighttime position takes a relatively long time, for example, about several seconds. The delay results because the optical filter switching apparatus 10 for use in the conventional surveillance camera 1 operating as described above performs the switching by means of screw connection between the gear rod 38, coaxially connected to the shaft of the stepping motor 35 and the downwardly opened inner screw 24 of the transfer driving guide 23 formed at the bottom of the optical filter holding section 21. Although there may be some difference depending on the combination of the number of revolution of the stepping motor 35 and the gear ratio between the gear rod 38 and the inner screw 24.
If the switching velocity of the optical filter holding section 21 is slow, the surveillance camera not only causes a gap of surveillance when switching the optical filter but also consumes more electric power because the driving time of the stepping motor is increased during the period of the switching operation. Accordingly, if the conventional surveillance camera 1 employs one or more batteries as a power source, the batteries will need to be frequently changed, which is a problem.
In addition, because the optical filter switching apparatus 10 has a screw connection structure, such as the gear rod 38, the downwardly opened internal screw 24, and other complex structures, the conventional camera 1 has problems of severe noise, complicated construction, and inferior endurance.
Furthermore, the optical filter switching apparatus 10 of the conventional surveillance camera 1 is not provided with a separate position sensing section for sensing the position of the optical filter holding section 21. Accordingly, if the power source is turned OFF due to a power failure or the like while the optical filter holding section 21 is being switched, the microprocessor 47 will recognize the optical filter holding section 21 as being located at the completely switched daytime or nighttime optical filter setting position. This results although the optical filter holding section 21 is stopped without having completely switched to the daytime or nighttime optical filter setting position.
In such a case, because the optical filter holding section 21 lies halfway between the daytime and nighttime optical filter setting positions even if the power source is restored, the surveillance camera 1 cannot normally filter light input through the lens 50. In addition, the motor 35 can be overloaded and damaged by the optical filter holding section 21 which displaces by an amount smaller than a preset amount of displacement when the stepping motor 35 is driven to change the modes, whereby the motor may be damaged.